Mutations in the NF1 tumor suppressor gene cause neurofibromatosis type 1 (NF1). NF1 encodes a GTPase activating protein (GAP) for p21ras (Ras) called neurofibromin, and studies by us and others have shown that the Ras signaling pathways are hyperactive in cells from NF1 patients. Neurofibromas are pathognomonic tumors that collectively affect 95% of NF1 patients. Neurofibromas are composed of Schwann cells, endothelial cells, fibroblasts, degranulating, inflammatory mast cells, and pericytes/vascular smooth muscle cells (VSMCs). Utilizing genetically engineered mice, Zhu et al (Science, 2002), found that haploinsufficiency of Nf1 in cells in the tumor microenvironment was required for development of neurofibromas. Our group previously provided the first genetic, cellular, and biochemical evidence that haploinsufficiency of Nf1 alters Ras activity and cell fates in mast cells. We have now shown that Nf1 mast cells release a diverse number of growth factors and other molecules in response to stimulation via c-kit ligand and that these growth factors and molecules collectively promote angiogenesis, the alteration of extracellular matrix and cell growth. In both Nf1 murine models and primary cells from NF1 patients, we have now identified haploinsufficient phenotypes in endothelial cells (ECs), VSMCs and fibroblasts that collectively promote neoangiogenesis, collagen synthesis and alter the extracellular matrix. Further, we have identified key growth factors and biochemical pathways that regulate the haploinsufficient gains-in-function in each of these lineages and verified that the murine model closely recapitulates the human disease phenotypes. Identification of these multiple Nf1 cellular phenotypes and development of in vitro and in vivo model systems provide us a platform to test compounds alone or in combination that inhibit key Ras effector kinases and growth factor receptors in neurofibromin deficient cells as a strategy to treat existing plexiform neurofibromas. Importantly, many of the kinases and molecular targets that we have discovered in neurofibromin deficient cells have also been identified in the treatment of more common cancers and provide us the opportunity to use existing preclinical compounds and phase 1-3 drugs that were developed for other uses. Therefore, in this application, we are focusing on compounds that have already beenextensively evaluated by pharmaceutical scientists in other model systems, thus allowing us to efficiently pursue multiple therapeutic targets. Finally, using a combination of PET and CT imaging, we now have the ability to image the development, growth, and metabolism of plexiform neurofibromas in genetically engineered mice in vivo as a function of time.